Fast full-field modulation transfer function analysis for photographic lens quality assessment.

Full-field modulation transfer function (MTF) data based on the slanted-edge method can give useful insights on the performance of a photographic lens sample and its shortcomings. Decentering and other out-of-tolerance states are recognized easily. A process to derive accurate lens MTF from slanted-edge spatial frequency response measurements is presented, covering chart design and alignment, data capture by standard digital cameras, slanted-edge algorithm implementation requirements, sensor and chart MTF corrections, and also visualization of the results. It is demonstrated that the reliability of the measured MTF values is by far good enough to support automated quality assessment with a measurement accuracy of ≈0.02 MTF and repeatability of ≲0.005 up to 100 c/mm. The measured full-field MTF values provide an unambiguous numerical criterion for comparison with expectations based on lens design.

Antiferromagnetic Ordering in the Single-Component Molecular Conductor [Pd(tmdt)2].

Crystals of [Pd(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate) were prepared in order to investigate their physical properties. The electrical resistivity of [Pd(tmdt)2] was measured on single crystals using two-probe methods and showed that the room-temperature conductivity was 100 S·cm(-1). The resistivity behaviors implied that [Pd(tmdt)2] was a semimetal at approximately room temperature and became narrow-gap semiconducting as the temperature was decreased to the lowest temperature. X-ray structural studies on small single crystals of [Pd(tmdt)2] at temperatures of 20-300 K performed using synchrotron radiation at SPring-8 showed no distinct structural change over this temperature region. However, small anomalies were observed at approximately 100 K. Electron spin resonance (ESR) spectra were measured over the temperature range of 2.7-301 K. The ESR intensity increased as the temperature decreased to 100 K and then decreased linearly as the temperature was further decreased to 50 K, where an abrupt decrease in the intensity was observed. To investigate the magnetic state, (1)H nuclear magnetic resonance (NMR) measurements were performed in the temperature range of 2.5-271 K, revealing broadening below 100 K. The NMR relaxation rate gradually increased below 100 K and formed a broad peak at approximately 50 K, followed by a gradual decrease down to the lowest temperature. These results suggest that most of the sample undergoes the antiferromagnetic transition at approximately 50 K with the magnetic ordering temperatures distributed over a wide range up to 100 K. These electric and magnetic properties of [Pd(tmdt)2] are quite different from those of the single-component molecular (semi)metals [Ni(tmdt)2] and [Pt(tmdt)2], which retain their stable metallic states down to extremely low temperatures. The experimental results and the band structure calculations at the density functional theory level showed that [Pd(tmdt)2] may be an antiferromagnetic Mott insulator with a strong electron correlation.

Surface aligned magnetic moments and hysteresis of an endohedral single-molecule magnet on a metal.

The interaction between the endohedral unit in the single-molecule magnet Dy_{2}ScN@C_{80} and a rhodium (111) substrate leads to alignment of the Dy 4f orbitals. The resulting orientation of the Dy_{2}ScN plane parallel to the surface is inferred from comparison of the angular anisotropy of x-ray absorption spectra and multiplet calculations in the corresponding ligand field. The x-ray magnetic circular dichroism is also angle dependent and signals strong magnetocrystalline anisotropy. This directly relates geometric and magnetic structure. Element specific magnetization curves from different coverages exhibit hysteresis at a sample temperature of ∼4 K. From the measured hysteresis curves, we estimate the zero field remanence lifetime during x-ray exposure of a submonolayer to be about 30 seconds.

Photochemistry of 6-amino-2-azido, 2-amino-6-azido and 2,6-diazido analogues of purine ribonucleosides in aqueous solutions.

The photochemistry of 6-amino-2-azidopurine, 2-amino-6-azidopurine and 2,6-diazidopurine ribonucleosides has been investigated in aqueous solutions under aerobic and anaerobic conditions. Near UV irradiation of 6-amino-2-azido-9-(2',3',5'-tri-O-acetyl-ß-D-ribofuranosyl)purine and 2-amino-6-azido-9-(2',3',5'-tri-O-acetyl-ß-D-ribofuranosyl)purine in the presence of oxygen leads to efficient formation of 6-amino-2-nitro-9-(2',3',5'-tri-O-acetyl-ß-D-ribofuranosyl)purine and 2-amino-6-nitro-9-(2',3',5'-tri-O-acetyl-ß-D-ribofuranosyl)purine. Under anaerobic conditions, both azidopurine ribonucleosides preferentially undergo photoreduction to 2,6-diamino-9-(2',3',5'-tri-O-acetyl-ß-D-ribofuranosyl)purine. The structures of the photoproducts formed were confirmed by UV, NMR and HR ESI-TOF MS spectral data. The photoproducts observed in this study for the aminoazidopurines are distinctly different from those observed previously for 6-azidopurine. When no amino group is present, the photochemistry of 6-azidopurine leads to the formation of a 1,3,5-triazepinone nucleoside. The energetics of the 6-nitreno moiety along both oxidation and ring expansion pathways was calculated using the nudged elastic band (NEB) method based on density functional theory (DFT) using DMol3. The role of the 2-amino group in regulating the competition between these pathways was elucidated in order to explain how the striking difference in reactivity under irradiation arises from the greater spin density on the 6-nitreno-9-methyl-9H-purin-2-amine, which essentially eliminates the barrier to oxidation observed in 6-nitreno-9-methyl-9H-purine. Finally, the importance of tetrazolyl intermediates for the photochemical activation of azide bond cleavage to release N2 and form the 6-nitreno group was also corroborated using the DFT methods.

Why does bromine square palladium off? An ab initio study of brominated palladium and its nanomorphology.

A first-principles description and prediction of brominated nanocrystals of Pd is presented. In particular, we conducted an extensive study of the adsorption behaviour of Br on various Pd surfaces (including both low and high Miller-index surfaces) as a function of its surface coverage. By coupling our calculated surface energies with ab initio (electrochemical) thermodynamics and the Gibbs-Wulff shape model, we find that the relative stability of the Pd surfaces is strongly modified by Br, allowing high Miller-index surfaces of Pd (namely the (210) surface) to become competitively favourable at moderate concentrations of Br. We also show that Pd nanoparticles assume a cube-like crystal shape at high concentrations of Br, exposing mainly the (100) facets with a Br surface coverage of 0.5 ML. This not only confirms and explains recent solution synthesis results, but also provides a quantitative atomic picture of the exposed surface facets, which is crucial in understanding the local surface chemistry of shape-controlled nanoparticles for better nanocatalyst design.

A first-principles study of ultrathin nanofilms of MgO-supported TiN.

As a first step towards a microscopic understanding of supported ultrathin nanofilms of TiN, we present state-of-the-art density-functional theory (DFT) calculations to investigate the interfacial properties of the TiN/MgO system as a function of film thickness. Optimized atomic geometries, energetics, and analysis of the electronic structure of the TiN/MgO systems are reported. In particular, we find that the work function of 1 ML of TiN(100) on MgO(100) exhibits a significant decrease, rationalized by the large surface dipole moment formation due to the changes in charge densities at the interface of this system. This decrease in the work function of TiN/MgO systems (as compared to pristine MgO(100) surface) could well benefit their application in metal-oxide-semiconductor devices as an ideal gate-stack material.

Tailoring magnetic anisotropy by graphene-induced selective skyhook effect on 4f-metals.

From macroscopic heavy-duty permanent magnets to nanodevices, the precise control of the magnetic properties in rare-earth metals is crucial for many applications used in our daily life. Therefore, a detailed understanding and manipulation of the 4f-metals' magnetic properties are key to further boosting the functionalization and efficiency of future applications. We present a proof-of-concept approach consisting of a dysprosium-iridium surface alloy in which graphene adsorption allows us to tailor its magnetic properties. By adsorbing graphene onto a long-range ordered two-dimensional dysprosium-iridium surface alloy, the magnetic 4f-metal atoms are selectively lifted from the surface alloy. This selective skyhook effect introduces a giant magnetic anisotropy in dysprosium atoms as a result of manipulating its geometrical structure within the surface alloy. Introducing and proving this concept by our combined theoretical and experimental approach provides an easy and unambiguous understanding of its underlying mechanism. Our study sets the ground for an alternative path on how to modify the crystal field around 4f-atoms and therefore their magnetic anisotropies.

Role of the Magnetic Anisotropy in Atomic-Spin Sensing of 1D Molecular Chains.

One-dimensional metal-organic chains often possess a complex magnetic structure susceptible to modification by alteration of their chemical composition. The possibility to tune their magnetic properties provides an interesting playground to explore quasi-particle interactions in low-dimensional systems. Despite the great effort invested so far, a detailed understanding of the interactions governing the electronic and magnetic properties of the low-dimensional systems is still incomplete. One of the reasons is the limited ability to characterize their magnetic properties at the atomic scale. Here, we provide a comprehensive study of the magnetic properties of metal-organic one-dimensional (1D) coordination polymers consisting of 2,5-diamino-1,4-benzoquinonediimine ligands coordinated with Co or Cr atoms synthesized under ultrahigh-vacuum conditions on a Au(111) surface. A combination of integral X-ray spectroscopy with local-probe inelastic electron tunneling spectroscopy corroborated by multiplet analysis, density functional theory, and inelastic electron tunneling simulations enables us to obtain essential information about their magnetic structures, including the spin magnitude and orientation at the magnetic atoms, as well as the magnetic anisotropy.

Light harvesting with multiwall carbon nanotube/silicon heterojunctions.

We report on a significant photocurrent generation from a planar device obtained by coating a bare n doped silicon substrate with a random network of multiwall carbon nanotubes (MWCNTs). This MWCNT/n-Si hybrid device exhibits an incident photon to current efficiency reaching up to 34% at 670 nm. We also show that MWCNTs covering a quartz substrate still exhibit photocurrent, though well below than that of the MWCNTs coating the silicon substrate. These results suggest that MWCNTs are able to generate photocurrent and that the silicon substrate plays a fundamental role in our planar device. The former effect is particularly interesting because MWCNTs are generally known to mimic the electronic properties of graphite, which does not present any photocurrent generation. On the basis of theoretical calculations revealing a weak metallic character for MWCNTs, we suggest that both metallic and semiconducting nanotubes are able to generate e-h pairs upon illumination. This can be ascribed to the presence of van Hove singularities in the density of states of each single wall carbon nanotube constituting the MWCNT and to the low density of electrons at the Fermi level. Finally, we suggest that though both MWCNTs and Si substrate are involved in the photocurrent generation process, MWCNT film mainly acts as a semitransparent electrode in our silicon-based device.

Electronic structure and oxygen vacancies in PdO and ZnO: validation of DFT models.

PdO is one of the most important catalytic materials currently used in the industry. In redox catalytic reactions involving PdO, the bulk phase is an additional source of oxygen. This leads to strong transformations not only at the surface of PdO but also in the near sub-surface and bulk regions. The redox process is, therefore, governed not only by the extent of PdO d-band filling, but also depends on the material properties of the PdO crystal--the ease with which its structure can be deformed. Methane oxidation is of key industrial interest, and therein the rate of CH(4) conversion depends strongly on the reversible oxygen defects formation on the surface and in the bulk of the catalyst. The present study gives a first insight into these complex phenomena at the atomistic level. Comparison of different density functional theory (DFT) approaches and their capacity to reproduce experimental values of the heat of formation as well as the band gap of the PdO are discussed in detail. Results from DFT calculations for an oxygen vacancy creation in the bulk and on the surface of PdO are presented and compared at the level of accuracy of the implemented approaches with defect calculations for ZnO. Many different modeling approaches based on functionals and pseudopotentials (non-modified PP and empirically tuned) have been evaluated in their aptness to capture key PdO properties. It was shown that simulations with the PP-115 pseudopotential gave the closest possible agreement to the relevant PdO thermodynamic data and energy of oxygen vacancy formation.

Reversible photoswitching between nitrito-N and nitrito-O isomers in trans-[Ru(py)(4)(NO(2))(2)].

Nitro-nitrito photoisomerisation is investigated in solid samples and solutions of trans-[Ru(py)(4)(NO(2))(2)]. Using light of wavelength 325 nm 50% of the N-bound Ru-NO(2) ligands can be switched to the O-bound Ru-ONO configuration (nitrito-N to nitrito-O isomerisation) at temperatures below T = 250 K in solids. The population of the isomeric configurations is determined with infrared spectroscopy from the decrease of the area of the nu(NO) stretching and delta(NO) deformation modes. In a frozen methanol-ethanol solution nearly 100% can be converted to the nitrito-O configuration. Upon heating above T = 250 K the Ru-NO(2) configuration is restored. The nitrito-O Ru-ONO configuration can be partially transferred back to the nitrito-N configuration by irradiation with light in the spectral range 405-442 nm. Using absorption spectroscopy on a frozen methanol-ethanol solution, two new bands at 447 and 380 nm are observed in the nitrito-O configuration compared to one at 334 nm of the nitrito-N ground state configuration. The photoconversion is initiated by the metal-to ligand charge transfer transition Ru(d) -->pi*(NO(2),py) as shown by the calculated partial density of states using Density Functional Theory. The calculations yield also the structure of the nitrito-N and nitrito-O isomer as well as the corresponding vibrational densities. The experimental structure of the ground state is determined using powder diffraction.

Photogeneration of nitrosyl linkage isomers in octahedrally coordinated platinum complexes in the red spectral range.

Octahedrally coordinated platinum nitrosyl complexes [Pt(NH(3))(4)(NO(3))(NO)](NO(3))(2) (1) and [Pt(NH(3))(4)(SO(4))(NO)](HSO(4))(CH(3)CN) (2) undergo linkage isomerization at temperatures below 130 K when excited with red light. Irradiation in the spectral range of 570-800 nm results in an inversion of the NO ligand from a Pt-NO to a Pt-ON configuration. The metastable state Pt-ON can be reverted back to the ground state (GS) Pt-NO by irradiation with blue-green or infrared light or by heating above 130 K. The characteristic shift of the nu(NO) stretching vibration from 1744 to 1815 cm(-1) in 1 and from 1714 to 1814 cm(-1) in 2 allowed the unambiguous identification of the respective nitrosyl isomers. Up to 26% of the complexes of 1 and 20% of 2 may be photochemically excited toward the metastable state (MS). Using X-ray crystallography and DFT calculations, it is shown that the Pt-NO in these {MNO}(8) complexes exhibits a bent arrangement with a Pt-N-O angle in the range of 117-120 degrees. As a consequence and in contrast to the known {MNO}(6) complexes only one metastable linkage isomer Pt-ON with a correspondingly bent Pt-O-N arrangement is formed, as evidenced by spectroscopy and DFT calculations. The calculated partial density of states shows that the charge transfer transition Pt(5d) --> pi(star)(NO) is responsible for the formation of the metastable state.

Quantum confinement effects in gallium nitride nanostructures: ab initio investigations.

We present ab initio density functional investigations of the atomic and electronic structure of gallium nitride nanodots and nanowires. With increasing diameter, the average Ga-N bond length in the nanostructures increases, as does the relative stability (heat of formation), approaching the values for bulk GaN. As the diameter decreases, the band gap increases, with the variation for the nanodots greater than that for the nanowires, in qualitative accordance with expectations based on simple geometrical quantum confinement considerations. Interestingly, in contrast to nanowires, the lowest unoccupied states of the nanodots exhibit an extended delocalized (Ga-derived) character, weighted in the centre of the nanodot.

Water adsorption on the stoichiometric and reduced CeO2(111) surface: a first-principles investigation.

We present a density functional theory investigation of the interaction between water and cerium oxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic structure and energetics of various configurations of water adsorption (for a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapor, employing the ab initio atomistic thermodynamics approach. Through our investigation, we obtain the phase diagram of the water-ceria system, which enables us to discuss the stability of various surface structures as a function of the ambient conditions. For the stoichiometric surface, we find that the most stable configuration for water is when it is bonded at the cerium site, involving two O-H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are introduced at the surface, which is predicted under more reducing conditions, the binding energy of water is stronger, indicating an effective attractive interaction between water molecules and oxygen vacancies. Water dissociation, and the associated activation energies, are studied, and the role of oxygen vacancies is found to be crucial to stabilize the dissociated fragments. We present a detailed analysis of the stability of the water-ceria system as a function of the ambient conditions, and focus on two important surface processes: water adsorption/desorption on the stoichiometric surface and oxygen vacancy formation in the presence of water vapor. A study of the vibrational contribution to the free energy allows us to estimate the effect of this term on the stability range of adsorbed water.

[Ru(py)4Cl(NO)](PF6)2.0.5H2O: a model system for structural determination and ab initio calculations of photo-induced linkage NO isomers.

Structure analysis of ground state (GS) and two light-induced (SI and SII) metastable linkage NO isomers of [Ru(py)4Cl(NO)](PF6)2.0.5H2O is presented. Illumination of the crystal by a laser with lambda = 473 nm at T = 80 K transfers around 92% of the NO ligands from Ru-N-O into the isomeric configuration Ru-O-N (SI). A subsequent irradiation with lambda = 980 nm generates about 48% of the side-on configuration Ru<(N)(O) (SII). Heating to temperatures above 200 K or irradiation with light in the red spectral range transfers both metastable isomers reversibly back to the GS. Photodifference maps clearly show the N-O configurations for both isomers and they could be used to find a proper starting model for subsequent refinements. Both metastable isomers have slightly but significantly different cell parameters with respect to GS. The main structural changes besides the Ru-O-N and RU<(N)(O) linkage are shortenings of the trans Ru-Cl bonds and the equatorial Ru-N bonds. The experimental results are compared with solid-state calculations based on density functional theory (DFT), which reproduce the observed structures with high accuracy concerning bond lengths and angles. The problem of how the different occupancies of SI and GS could affect refinement results was solved by a simulation procedure using the DFT data as starting values.

The Creutz-Taube complex revisited: DFT study of the infrared frequencies.

The structural parameters, electronic properties, and infrared frequencies of three binuclear ruthenium complexes, [(NH(3))(5)Ru(pyrazine)Ru(NH(3))(5)](n+), n = 4-6, have been investigated with density functional theory. Structural analysis demonstrates that the structure of the mixed-valence 5+, or [II,III], is not an intermediate of the reduced 4+, or [II,II], and the oxidized 6+, or [III,III], compounds. Electronic structure comparison shows that the Ru d(yz) antibonding orbital is empty when n = 5 and 6 and occupied, when n = 4. The infrared frequencies have been calculated for a sequence of models with increasingly detailed accounts of counterions, ranging from the free Creutz-Taube 5+ ion, over jellium embedded, COnductor-like Screening MOdel (COSMO), to the experimental structure of the triclinic [II,III](tos)(5).4H(2)O (tos = p-toluenesulfonate) crystal. Analysis of the Ru vibrations shows that the spectra for the two symmetry-inequivalent Ru atoms are essentially the same. We find that Ru-Ru modes exist near three well-defined frequencies in the solid: at 145, 285, and 345 cm(-1). Similar results are also obtained for the simplified jellium and COSMO models. The spectral properties of these vibrational correlations testify to the existence of two coupled Ru atoms in the same charge state.

Vibrational spectroscopy of the double complex salt Pd(NH3)4(ReO4)2, a bimetallic catalyst precursor.

Tetraamminepalladium(II) perrhenate, a double complex salt, has significant utility in PdRe catalyst preparation; however, the vibrational spectra of this readily prepared compound have not been described in the literature. Herein, we present the infrared (IR) and Raman spectra of tetraamminepalladium(II) perrhenate and several related compounds. The experimental spectra are complemented by an analysis of normal vibrational modes that compares the experimentally obtained spectra with spectra calculated using DFT (DMol3). The spectra are dominated by features due to the ammine groups and the ReO stretch in Td ReO4-; lattice vibrations due to the D4h Pd(NH3)42+ are also observed in the Raman spectrum. Generally, we observe good agreement between ab initio calculations and experimental spectra. The calculated IR spectrum closely matches experimental results for peak positions and their relative intensities. The methods for calculating resonance Raman intensities are implemented using the time correlator formalism using two methods to obtain the excited state displacements and electron-vibration coupling constants, which are the needed inputs in addition to the normal mode wave numbers. Calculated excited state energy surfaces of Raman-active modes correctly predict relative intensities of the peaks and Franck-Condon activity; however, the position of Raman bands are predicted at lower frequencies than observed. Factor group splitting of Raman peaks observed in spectra of pure compounds is not predicted by DFT.

Quantum chemical analysis of electronic structure and n- and p-type charge transport in perfluoroarene-modified oligothiophene semiconductors.

Density-functional theory (DFT) is employed to investigate the structural, electronic, and transport properties of several isomeric fluoroarene-oligothiophene-based semiconductors. Three oligothiophene systems varying in the perfluoroarene group positions within the molecule are studied to understand the electronic structure leading to the observed mobility values and to the n- or p-type behavior in these structures. Analyses of both intermolecular interactions in dimers and extended interactions in crystalline structures afford considerable insight into the electronic properties and carrier mobilities of these materials, as well as the polarity of the charge carriers. From the calculated carrier effective masses, we find that sterically governed molecular planarity plays a crucial role in the transport properties of these semiconductors. Our calculations correlate well with experimentally obtained geometries, highest-occupied molecular orbital (HOMO)/lowest-unoccupied molecular orbital (LUMO) energies, and the experimental carrier mobility trends among the systems investigated.

Out-of-Plane Alignment of Er(trensal) Easy Magnetization Axes Using Graphene.

We have studied Er(trensal) single-ion magnets adsorbed on graphene/Ru(0001), on graphene/Ir(111), and on bare Ru(0001) by scanning tunneling microscopy and X-ray absorption spectroscopy. On graphene, the molecules self-assemble into dense and well-ordered islands with their magnetic easy axes perpendicular to the surface. In contrast, on bare Ru(0001), the molecules are disordered, exhibiting only weak directional preference of the easy magnetization axis. The perfect out-of-plane alignment of the easy axes on graphene results from the molecule-molecule interaction, which dominates over the weak adsorption on the graphene surface. Our results demonstrate that the net magnetic properties of a molecular submonolayer can be tuned using a graphene spacer layer, which is attractive for hybrid molecule-inorganic spintronic devices.

Strong antiferromagnetic exchange between manganese phthalocyanine and ferromagnetic europium oxide.

We report on the antiferromagnetic exchange coupling between a submonolayer of Mn(II)-phthalocyanine molecules and a ferromagnetic Eu(II)-oxide thin film. The exchange energy is larger by nearly two orders of magnitude compared to previous studies involving oxidic substrates.